The invention relates to a method of the continuous digesting of cellulosic fiber material (i.e. wood chips or other finely commuted raw material for pulp production). In particular, the method of the invention is directed to continuous cooking methods wherein the heat necessary to bring the fiber material up to process temperature is provided by direct or indirect steam addition to a digester vessel at super atmospheric pressure.
Conventional continuous digesters having a steam phase at the top thereof are disclosed in U.S. Pat. Nos. 3,532,594 and 3,802,956. In such conventional structures, normally an impregnation vessel is provided in which a steam zone is maintained at the top of the vessel, the steam being added to the vessel above the liquid level therein but under the fiber material level therein (see conduit 19 in U.S. Pat. No. 3,532,594) in order to expel the air from the fiber material, which air is then released at the top of the vessel. After this pretreatment, the fiber material is fed to the top of a digester vessel in which a steam phase is also maintained above the level of fiber material and cooking liquid. Conventionally, steam for further heating of the fiber material is added in the steam phase of the digester vessel at the top of the digester vessel (see line 61 in U.S. Pat. No. 3,802,956). Steam which is consumed by condensation on the introduced fiber material (which is colder) is continuously replaced by added new steam so that at all times a steam phase is maintained in the digester top.
In such prior art structures, there is a tendency for the heating of the steam to be uneven across the digester cross-section, which is a great disadvantage in the digesting process since the degree of delignification is strongly dependent upon the temperature. Portions of the fiber material which are subjected to too much heat can be overcooked while portions which receive too little heat will be undercooked and will come out as reject material when screening of the digested pulp which has been removed from the digester takes place. Thus, the yield of suitable pulp per unit of fiber material is not as great as desired. One reason for this uneven heating is the tendency for the fiber material to form a pile configuration in the digester vessel.
According to the present invention, a method for continuously digesting cellulosic fiber material is provided wherein the amount of reject material in the digested pulp can be greatly reduced, thus increasing yield calculated on quantity of introduced fiber material. According to the method of the present invention, a standing vessel having a fiber level therein, a liquid level therein, and a steam phase above the fiber and liquid levels is provided. The method comprises the steps of continuously feeding cellulosic fiber material into the vessel downwardly through the steam phase to establish the fiber levels, and continuously feeding the material downwardly in the vessel; adding digesting liquid into the vessel to maintain the liquid level therein; effecting spreading of the downwardly falling fiber material in the steam phase over the vessel cross-section and feeding the downwardly falling fiber material in the steam phase by introducing a major portion of the steam to be introduced into the steam phase at a first vertical position about 0.0 to 100 cm. above the higher of the fiber and liquid levels in the vessel; and withdrawing digested pulp from the bottom of the vessel. The withdrawn digested pulp has less than about two percent reject material as compared to ten to twelve percent which is common in the prior art. The spreading and heating step is accomplished by spreading the fiber material so that it is substantially evenly distributed over the vessel cross-section so that the configuration of the fiber level in the vessel is substantially a horizontal plane. This may be accomplished by addition of steam above the first vertical position, at a second vertical position, to initially effect the spreading, the substantial portion of the heating being accomplished by the steam introduced at the first vertical position countercurrent to the fiber material flow.
It is the primary object of the present invention to provide a method for digesting cellulosic fiber material in a digesting vessel having a steam phase which has a low level of reject material in the digested pulp withdrawn from the digester. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.